134. SOME PRACTICAL LESSONS FROM SOIL BACTERIOLOGY 
been learned, this should be some one of the legumes, since this 
family of plants alone assimilates nitrogen from the air, at least 
in any considerable quantity. Other plants have been used for 
the purpose, but while they are valuable in supplying some organic 
material which helps maintain the store of humus, they are far 
inferior to legumes which, in addition to all the other advantages, 
add usable nitrogen in quantity. Attention should be given to the 
nature of the soil, and to the kind of legume that will best flourish 
in the soil; the legume must produce plenty of root tubercles, 
otherwise the chief value of the green manuring is lost. Green 
manuring is of particular value in sandy, loose soils, where the 
humus is scanty, and where the texture of the soil facilitates 
losses by draining. In such soils so rapid is the draining that it 
is sometimes difficult to get fertilizers to remain in the soil long 
enough for their proper assimilation by the plant. The use of 
legumes, plowed under to furnish a mass of decaying vegetation, 
greatly improves the texture of the soil and will, in time, give 
them a fair humus content. By this means very unpromising 
sandy soils can be reclaimed to a fair condition of fertility. The 
legumes found to be best adapted to such sandy soils are the cow 
pea, the soy bean, the velvet bean and the crimson clover. 
With clay soils, on the other hand, green manuring must be 
handled somewhat differently. The density of the soil reduces 
the ordinary losses by drainage, so that there is less need of the 
green manuring. The legumes found most useful on such lands 
are lupins, seradella, yellow, red and crimson clover, field peas, 
horse beans and veiches. Because of the density of the soil, de- 
composition does not progress so freely; hence care must be taken 
not to overdo the treatment by plowing in too much of the green 
plant. 
The extent of the utilization of the legume after growing, de- 
pends upon the completeness of the decomposition and the even- 
tual nitrification of the material that is plowed under. It is 
possible to plow under such a large quantity of vegetable matter 
